Diquafosol is a purinergic receptor agonist also called P1,P4-di(uridine-5′)tetraphosphate or Up4U, and is known to have an effect of stimulating secretion of tears as disclosed in Japanese Patent No. 3652707 (PTD 1). Cornea, 23(8), 784-792 (2004) (NPD 1) describes that instillation of an ophthalmic solution comprising diquafosol tetrasodium salt has improved corneal epithelium disorder of dry eye patients. In our country, an ophthalmic solution comprising diquafosol tetrasodium salt at a concentration of 3% (w/v) is actually used as a remedy for dry eyes (product name: DIQUAS (registered trademark) ophthalmic solution 3%).
As for an ophthalmic solution, it is necessary for the solution to have physicochemical properties that are stable during the courses of production and distribution as well as the course of storage by a patient. In particular, regarding such an ophthalmic solution as the one in which precipitates are formed during the course of distribution or during storage by a patient, the precipitates cannot be removed afterward, and therefore such an ophthalmic solution is undesirable for use as an ophthalmic solution. Although precipitates formed in an ophthalmic solution during the course of its production can be removed in the process of filtration sterilization of the ophthalmic solution, a filter is clogged during the filtration to accordingly deteriorate the efficiency of filtration sterilization, resulting in a problem of an increase of the production cost.
As to a method for inhibiting formation of precipitates in an ophthalmic solution, Japanese Patent Laying-Open No. 2007-182438 (PTD 2) discloses a method according to which glycerin is added to the ophthalmic solution, for example. As described in this document, the properties and/or the state of precipitates vary depending on the type of active ingredient and/or the type of additive, and accordingly the method for inhibiting formation of precipitates varies depending on the ophthalmic solution.